The present invention generally relates to an internal combustion engine with a decompression device, such as a small air-cooled two-cycle gasoline engine, especially adapted for a portable or conveyable working machine, such as a chain saw, trimmer, etc. and more particularly, to an automatic decompression device which is mounted to a cylinder portion of the internal combustion engine.
Generally, in such portable or conveyable working machines, an air-cooled two cycle gasoline engine with a recoil starter is used as a power source, which provides reduction in size and weight thereof, and in many cases, the decompression device is mounted to the engine so that the start-up of the engine is facilitated. The foregoing decompression device basically includes a perforation passage which allows a combustion chamber, defined by an inner wall of a cylinder of an engine body, to communicate with the outside, a pressure reducing valve for closing the passage mounted therewithin, and a valve operating member for opening the pressure reducing valve responsive to the start-up operation (cranking period).
Various attempts have been made to provide decompression devices which reduce the pressure inside the cylinder of the engine when starting the engine. In such engines for conveyable working machines, such as a chain saw, a manually operated decompression device, which is to be manually operated every time the engine is started, is normally used. This is because it is necessary for the construction thereof to be simple, compact and sturdy in order to easily operate.
Further, other various attempts have been made to come up with an automatic decompression device which is interconnected to a recoil starter device of the engine. For example, Japanese Utility Model Laid-Open Publication No. Hei 2-139366 teaches an automatic decompression device wherein the swing motion of a swinging lever, responsive to the rotation of a recoil starter, operates an exhaust valve of the decompression device to open so that the air-fuel mixture in a cylinder is released to the outside and the pressure of the combustion chamber can be reduced. Other references, such as Japanese Utility Model Laid-Open Publication No. Hei 2-144665, discloses an automatic decompression device which includes a swinging lever, the tip portion thereof being swung upon contact with a recoil rope of the recoil starter when the recoil rope is drawn, and a push rod reciprocating in accordance with the swing motion of the swinging lever, wherein a pressure reducing valve of the decompression device is made to open by the reciprocation of the push rod so that a gas within the cylinder is released to the outside thereby reducing the pressure of the combustion chamber.
In a conventional decompression device, as described above, if the operating mechanism portion of the pressure reducing valve thereof is exposed and, especially in the case of being adapted for an engine of conveyable working machines, such as a chain saw, it is likely that the operating mechanism portion of the pressure reducing valve of the decompression device will contact external obstacles, such as for example, trees, during operation. This results in the operating mechanism portion of the pressure reducing valve being damaged thereby causing a malfunction thereof. In the manually operated decompression device, because the pressure reducing valve is kept closed before the start-up thereof, it is necessary to push in the pressure reducing valve by hand every time the engine is started so that the gas within the cylinder of the engine can be released. Further, the foregoing conventional automatic decompression device, interconnected to the recoil starter of the engine, has problems in that the structure thereof is complicated and that enough room is needed to allow large displacement of the moving member, such as the swinging lever.
In view of the foregoing disadvantages inherent in the known types of prior art, the present invention solves the above-mentioned problems. Thus, it is an object of the present invention to provide an automatic decompression device, which can be adapted for the internal combustion engine for conveyable working machines, such as a chain saw, wherein the operating mechanism portion of the pressure reducing valve of the decompression device cannot be damaged by contacting external obstacles, such as for example, trees while at work, and to provide the same which is capable of automatically reducing the pressure of the combustion chamber within the cylinder in association with the start-up of the engine, wherein the structure is simplified and compactly made.